Characterization of Colletotrichum Populations from Watermelon in Virginia

Abstract

Watermelon anthracnose, caused by Colletotrichum orbiculare, is a significant threat to watermelon production in the southeastern United States, with increasing concerns regarding reduced cultivar performance and declining efficacy of several fungicide classes. Field trials conducted in Painter, VA during 2024 and 2025 evaluated the susceptibility of ten commercially available seedless watermelon cultivars and the efficacy of 11-13 fungicides representing multiple modes of action. In parallel, fungicide sensitivity assays were conducted on 241 Colletotrichum isolates collected from symptomatic watermelon tissue across Southampton County, VA to characterize sensitivity to difenoconazole and thiophanate-methyl. Isolates were identified as either C. orbiculare or C. sojae, a species not previously associated with watermelon anthracnose in U.S. production systems. All cultivars developed severe disease, exceeding 90% final foliar severity. Fungicide efficacy varied among chemistries. Mancozeb and chlorothalonil consistently provided the greatest reductions in final foliar disease severity and standardized area under the disease progress curve (sAUDPC). Demethylation inhibitor (DMI; FRAC 3) triazole fungicides were either ineffective in 2024 or less effective than most products in 2025, whereas the DMI triazolinone, prothioconazole, provided disease suppression comparable to chlorothalonil in both years. Quinone outside inhibitor (QoI; FRAC 11) and methyl benzimidazole carbamate (MBC; FRAC 1) fungicides, applied alone or in premixtures, reduced disease severity relative to the nontreated control. Post harvest evaluations in 2025 revealed high fruit infection in several treatments, while mancozeb, chlorothalonil, thiophanate methyl, and pyraclostrobin maintained low (<5%) postharvest disease levels. Sensitivity assays revealed significant differences in responses among species-fungicide combinations. For difenoconazole, elevated EC50 values in C. orbiculare were found in 2024, whereas EC50 values in C. sojae displayed the opposite trend, with more elevated values found in 2025. For thiophanate-methyl, elevated EC50 values increased in C. orbiculare in 2025, while C. sojae remained relatively stable across years. Collectively, these results demonstrate the limited cultivar resistance and variability in fungicide sensitivity among regional Colletotrichum populations but also identified several effective fungicide options for anthracnose management in Virginia watermelon production systems.